In industrial manufacturing of products, for many years now, the product quality has played an increasingly important role. On the one hand, high product quality can be achieved by correspondingly designed and stable manufacturing processes. On the other hand, the quality parameters of a product must be inspected in a manner as reliable and complete as possible in order to detect quality deficiencies at an early stage. In many cases, the quality of a product surface plays a role. This may involve decorative surfaces such as painted surfaces on automobiles or household items, or technical surfaces such as surfaces of finely worked metallic pistons or bearing surfaces.
There are already many suggestions and concepts for automatically inspecting reflecting surfaces. Often, however, the known methods and apparatus can only be used for a specific application because they require a high a priori knowledge of the surface to be inspected. Furthermore, the known methods and apparatus are not sufficiently perfected to enable an efficient and reliable inspection of surfaces under industrial conditions. In this case, industrial conditions refer to the compliance with cycle times, which are relevant for inclusion in industrial manufacture, the ability to perform the surface inspection in a manufacturing hall, and/or the possibility to simply and quickly adapt the surface inspection to changing products.
Consequently, for example, in the automobile industry, up until today, a visual inspection of paint surfaces is performed to a significant degree by experienced and trained persons. The degree of automation in the inspection of reflective painted surfaces is substantially lower than the degree of automation in the manufacture itself. An example of an apparatus for the visual inspection of the painted surface of a motor vehicle is described in U.S. Pat. No. 5,636,024. The apparatus includes a tunnel through which the motor vehicles with the painted surfaces to be inspected are transported. There are light sources on the inner walls of the tunnel which generate a striped pattern of light and dark stripes. These striped patterns are reflected by the painted surface of the motor vehicle. The inspection of the painted surface is carried out by persons who are standing in the tunnel and are visually checking the reflections of the striped pattern on the painted surface. It is easily comprehensible that such a procedure to a large extent depends on the skills of the observer and thus provides limited reliability only. Furthermore, such a procedure is work intensive and accordingly expensive.
German patent publication 103 17 078 A1 describes a deflectometric method and a corresponding apparatus. In this method, a striped pattern with a sinusoidal brightness gradient is projected onto a screen which is arranged obliquely to the surface to be inspected. The projected pattern is changed or moved so that correspondingly changed striped patterns hit the surface. While or after the changing/moving of the pattern an image of the surface with the reflected pattern is recorded. Through a mathematical linking of the images taken at different points in time, a result image is to be generated on the basis of which areas of the surface having defects and areas of the surface devoid of defects can be mathematically or visually differentiated. A similar method and a similar apparatus are known from a publication by Sören Kammel entitled: “Deflectometry for the Quality Testing of Specularly Reflecting Surfaces”, published in the German magazine tm-technisches Messen, edition April, 2003, pages 193 to 198. In this case, an evaluation of the obtained image data is carried out by comparison to a reference, which requires an exact alignment of the surface to be examined to the reference data.
Further methods and apparatus for the optical inspection of at least partially reflecting surfaces are disclosed in German patent publication 198 21 059 C2 or in U.S. Pat. 6,100,990. Here too, striped patterns with sinusoidal brightness gradient across the surface to be inspected are observed. In all cases, the striped patterns are created on a screen which is arranged at an angle to the surface to be inspected. These methods thus have the disadvantage that only a relatively small surface can be inspected, which in addition must be arranged in an at least mostly known and defined position and alignment to the striped pattern. A quick, reliable and efficient inspection of reflecting surfaces under industrial conditions is not possible therewith.
U.S. Pat. Nos. 5,726,706 and 4,918,321 each disclose a method and apparatus wherein a motor vehicle is passed under a bridge-like arrangement on which a plurality of cameras are arranged. The detection of paint defects or other surface defects is done with the aid of light stripes or light bands whose reflection is analyzed. In a defect-free surface, each camera sees the respective light or dark stripes. A surface defect, such as a dent, results in the light to be diverted from a light stripe into the image of a dark stripe so that a bright light point is visible in the image of the dark stripe. These methods have limited detection rates. Small scratches or duller paint areas which do not create any considerable reflections in any other direction in space but the surrounding areas cannot be detected with these apparatus.
German patent publication 10 2005 038 535 A1 recognized the problems encountered with the adaptation of the surface inspection to the cycle times of industrial manufacturing and suggests a rotation-symmetrical, in particular cylindrical stripe projector for illuminating an object having a surface to be inspected. A cylindrical hollow body has its inner wall provided or coated with an electroluminescent foil. The foil is to be provided with colored or gray-scale stripes which are either printed thereon or realized with the help of a second foil. The cylindrical hollow body is to be mounted in a second, outer hollow body in such a manner that it can be mechanically put into a rotational movement. The rotational movement is to generate the change in the striped pattern relative to the surface to be inspected. The mechanical movement of the cylindrical hollow body, however, constitutes a disadvantage of this concept, in particular, if the apparatus is to be used for the inspection of objects with large surfaces.